He was 14 years old when this happened. It was a youthful lapse of judgement.
His mother took him to a tattoo shop and lied about his age to the owner so he could get a tattoo. Then she helped her son cover up a murder and got her other son to lie in court. This is the woman who raised him, so why is it surprising?
I think 'youthful lapse of judgement' should go down as English understatement. It's more than a lapse of judgement. It is not even an ordinary murder. You could rationalise a situation where he is jealous and in a fit of rage, kills her, but this is so much more. He has basically butchered her and tied her up, perhaps with the intention (the idea occurring to him on the spur of the moment) that he needed to make the murder look non-spontaneous in order to deflect suspicion from himself.
His mother is bound to defend him. I would die for my children, though I'm not sure if I would actually lie in court in these circumstances. I would go a long way for them though, I can tell you that. Maybe Luke is the killer but he lied to his mother and brother and told them he had been waiting around for Jodi, so had no alibi as such, and they needed to cover for him otherwise he would be framed? It's common for close relatives and loved ones to lie in court for that reason, hence the perjury warning that the trial judge gave both the mother and the brother, Shane Mitchell. In the old days, the judge would not have even bothered doing that, he would have just told the jury to treat their evidence with due caution.